The present invention relates generally to thermal circulation gas treaters and more particularly to a thermal circulating gas treater having a system for controlling and optimizing the gas temperature in a high pressure separator. The effluent from a petroleum or gas well may contain oil and gas, with some of the oil and/or water emulsified with the gas, and under certain conditions, with the water forming hydrates. A common way to treat the connate well fluids has been to heat them to a predetermined temperature, to aid in breaking the emulsions and hydrates for the separation of gas from the oil or liquified hydrocarbons at the separation temperature. The heating of the well streams is usually performed at or near the well head by equipment that must be automatic, as the well heads may only be rarely visited by the producer's workers. Some problems occur with the heating of well streams, as a controlled temperature is highly desirable, and overheating or underheating are detrimental to the treatment process.
One method of treating the heated well stream is to pass it through a high pressure separator. The high pressure separator generally comprises an elongate closed tank wherein effluent introduced at an upper portion of one end thereof is separated into a separator liquid which collects in a lower portion of the tank and a gas which collects above the separated liquid. It is conventional to control the temperature of the liquid collected in the separator by a separator heating coil. The separator heating coil is located in a lower portion of the separator where it is immersed in the separator liquid and supplies heat thereto. A heating medium flowing through the separator heating coil is heated by a heating unit. The amount of heat provided to the separator liquid by the separator heating coil is dependent on the flow rate and temperature of the heating medium through the coil. Various means may be employed controlling this heating medium flow rate, such as a controlled heat siphon as described in U.S. Pat. Nos. 4,342,572 and 4,198,214 of Rodney T. Heath which are hereby incorporated by reference for all that is contained therein.
It is also conventional for separator heat control systems to control the temperature of the effluent entering the separator by preheating the effluent. One method of preheating the effluent is to pass it through a coil in a heated liquid bath. Apparatus of this type is described in U.S. patent application Ser. No. 537,298 filed Sept. 29, 1983 by Rodney T. Heath, which is hereby incorporated by reference for all that it contains. Other methods such as conventional pumping, etc. may also be used in systems.
A supply (control and fuel) gas system is provided which provides fuel to the heater burner and which also provides control gas to various separator controls. The supply gas is supplied by gas from the high pressure separator which must undergo substantial pressure reduction from line pressure of e.g. 1500 psig to a gas supply pressure of about e.g. 30 psig to 1 psig. This pressure reduction produces substantial cooling in the supply gas and thus the supply gas is passed through a coil in the heater bath to prevent it from freezing as a result of the pressure reductions. It is generally desirable to maintain the liquid bath in the heater at an elevated temperature, especially in extremely cold weather conditions to prevent the supply gas regulators from freezing.
It is further desirable to maintain the heat of the liquid bath in the heating unit at a relatively high temperature for heating efficiency. However, the temperature/pressure head of well effluent passing through this type of heating coil is variable, due to well head flowing conditions and weather conditions. In order to compensate for these differences in well effluent head and maintain a constant separation temperature, conventional units have varied the temperature of the liquid bath in which the effluent heating coil is immersed. A problem with this type of control is that it is rather slow in response due to the time that it takes to heat or cool the liquid bath. Another problem is that varying the temperature of a large liquid bath over a wide temperature range is not thermally efficient. Still another problem is that if the bath temperature becomes too low, there is not enough heat in the bath to treat the hydrocarbons and water which may be contained in the separator liquid bath or to prevent possible freezing of the associated supply gas system.